1. Field of the Invention
The present invention relates to a method and apparatus for producing an oxide thin film, and more particularly to a method and apparatus for producing an oxide thin film having a metal element in its composition.
2. Description of the Prior Art
Conventionally, as a method of producing oxide thin films containing a metal element (grouped into oxide semiconductors, insulating materials, oxide superconductors, oxide magnetic substances, and others by the properties of the films), there have been widely put in practice a reactive vapor deposition method, a sputtering method, a chemical vapor deposition method, and the like. For example, according to the reactive vapor deposition method, a specified metal element is evaporated from a vapor source in an oxygen gas environment at a pressure in the range of about 10.sup.-5 to 10.sup.-3 Torr in a vacuum chamber so that an oxide of the evaporated metal element is deposited directly onto a substrate.
According to another known production method, after depositing a metal thin film onto a substrate in a vacuum chamber, an oxide thin film is obtained by taking out the substrate out of the vacuum chamber and oxidizing the metal thin film, or by introducing oxidization gas into the vacuum chamber and oxidizing the metal thin film.
In either of the above-mentioned methods, the vapor depositing or growing process starts inside the vacuum chamber at a pressure of not smaller than 10.sup.-8 Torr.
However, since the vapor depositing or growing process starts at a pressure of 10.sup.-8 Torr or above in either of the above-mentioned conventional methods, residual gases such as H.sub.2 O, N.sub.2, H.sub.2, CO.sub.2, or the like are taken in as impurities, which results in reducing the purity of the oxide thin film produced.
According to the above-mentioned reactive vapor deposition method, the vapor deposition process is carried out in an approximately 10.sup.-5 to 10.sup.-3 Torr oxygen gas environment, and disadvantageously the vapor source is oxidized in the oxygen gas environment. Therefore, it is very difficult to obtain a stable or constant vapor rate and accurately control the composition of the resulting oxide thin film. Furthermore, since the degree of vacuum is not so good, i.e., the pressure inside the vacuum chamber is high, the vapor source must be heated to a high temperature to increase the vapor pressure. For example, when using yttrium (Y), a metal element known as having a high melting point, a temperature of approximately 1,100.degree. C. is necessary to evaporate the metal element at a pressure of 10.sup.-6 Torr. Therefore, in order to vaporize metal having a high melting point, there is a limitation in sort of the vapor source to be adopted, and an evaporation cell such as a Knudsen cell or the like having an excellent controllability of vapor rate cannot be used.
On the other hand, according to the thin film producing method of firstly depositing a metal thin film and thereafter oxidizing the same, a residual gas covers the film surface in a short time (approximately 100 seconds) in a vacuum condition of approx. 10.sup.-8 Torr to reduce the crystallinity and surface activity of the metal thin film. The above results in the problem that no oxide thin film having an excellent crystallinity and purity can be obtained after the oxidization process. Furthermore, once the substrate with a metal thin film is taken out of the vacuum chamber, the film surface will be contaminated, resulting in further reduction of the crystallinity and purity. When an oxidizing gas is introduced into the vacuum chamber to oxidize the metal thin film, the oxidizing gas is absorbed into the internal walls of the vacuum chamber. Therefore, when producing oxide thin films repeatedly, it is difficult to evacuate the vacuum chamber to a high vacuum, which fatally results in reducing the productivity.